Foam cleaning is a chemical cleaning process for removal of water hardness scales, corrosion deposits, pre-operational deposits and organic foulants from industrial equipment. A cleaning solvent is employed that is selected to satisfactorily dissolve the objectionable scale or foulant. A surfactant is added to the solvent to promote foaming, and the resulting solvent/surfactant liquid is foamed with a gas to generate a much larger volume of foam. Forcing a gas under pressure into the body of the liquid is one method that has been used to create foam, which is then introduced into the equipment to be cleaned by the constant pressure of the gas. The foam is allowed to flow through the vessel or process equipment to be cleaned. In the conventional process, the foam is passed through the equipment only one time. Apparatuses for foam generation for use in methods for chemical foam cleaning of vessels are described in U.S. Pat. No. 3,212,762 to Carroll, et al. and U.S. Pat. No. 4,133,773 to Simmons.
The foam cleaning process is particularly useful for cleaning such equipment as gas lines which cannot support the weight of liquids, condensers used in the utility industry which employ thousands of tubes, and other heavy-duty equipment such as tanks, coils, tubing and the like.
A disadvantage of the conventional method of employing foamed solvents is that it is wasteful. Since chemical cleaning is dependent on reaction rate and time, much of the foamed solvent which flows through the equipment remains unreacted. Additionally, the "used" foamed solvent may present an environmentally undesirable waste product and, since the conventional once-through process also requires a large quantity of foamed solvent, a large quantity of waste is generated that may present a disposal problem.
Another industrial cleaning problem is the decontamination of equipment which may contain radioactive material. While foamed liquids are effective for cleaning foulants from many types of structures, it is desirable to generate as little waste product as possible in the cleaning operation because the disposal of radioactive cleaning material must be carefully monitored. It is further of interest to reduce the release of gases in the cleaning process for environmental reasons.
There have been attempts to reuse foamed solvent after it has been employed for cleaning. However, prior methods have not been successful in efficiently using solvent and concurrently reducing waste product.
One prior method for reusing foamed solvent requires the use of a collection vessel or reservoir which is open to the atmosphere. When foam is introduced to the vessel being cleaned and begins to react on the scale to be removed, it begins to naturally break-down into a two-phase composition as a gas and a liquid. The term "half-life" is used to describe the amount of time it takes to recover one-half of the original volume of liquid from a measured volume of foam. In the conventional process, the foam is passed through the vessel, then the remaining foam, as well as the break-down products from the foam, are discharged into the collection vessel. The foam can be further broken into its separate liquid and gas components by use of a chemical substance or admixture known as "foam breaker" or an "antifoam." The gas thus generated, termed "break-out gas" is allowed to escape to the atmosphere. The liquid thus generated, in combination with the natural break-down liquid of the foam, is termed "break-out liquid." The "break-out liquid" is pumped to a foam generator where it is mixed with additional nitrogen or air for foam regeneration. This break-out liquid, since it is contaminated with antifoam materials, is more difficult to foam and additionally has a shorter half-life then the original foam; therefore, additional foaming agent or stabilizers must usually be added in order to sustain good foam stabililty. Break-out liquid that is severely contaminated with antifoaming agent may not refoam. A collection tank of sufficient size is required in this process, since it is not unusual for foam to rapidly fill the collection vessel and overflow, causing pollution to the immediate area.
Another method for reusing solvent employs a collection tank of sufficient size to allow foam breaking to occur without the use of an antifoam. This method requires an even larger collection tank than the method in which antifoam is used, because the foam occupies a larger volume when not chemically broken. This method is disadvantageous as it is often difficult to predict how large of a collection tank will be required for a particular job. Even if this can be predicted, space constraints at the job site may preclude employment of a sufficiently large tank.
Another prior art attempt to reuse foamed solvent was disclosed by Crowe, et al., in U.S. Pat. No. 3,436,262. The '262 system employs a liquid comprising a solvent in combination with a foaming agent and a foam stabilizer that is foamable by the use of heat. The foaming agent must be gaseous in the foaming unit and liquid in the collapsing unit according to the '262 patent. A heating unit must be employed to cause the liquid to foam. After the foam is passed through the vessel to be cleaned, it is collected in a foam collapsing unit where it is cooled to at least 60.degree. F., which cooling condenses the foam into a liquid. The liquid can then be heated to form a foam. The Crowe et al. method is suitable for some applications, but is disadvantageous due to the requirement for the foaming agent and stabilizer, the need to heat and cool the foam and the need to break the foam completely before refoaming.
Thus, prior art attempts to reuse solvent followed the procedure of breaking down foam completely, and attempting to refoam it.
Prior art methods of using and reusing foamed solvents generally require the addition of chemicals such as antifoam, additional foaming agent, and or foam stabilizers. Therefore, the attempt to reuse the solvent may not be economically advantageous because of the added cost of the necessary treating chemicals. The conventional once-through process also may require costly quantities of inert gas when inert gas is needed for the application.
Present methods for employing foamed solvents for cleaning also undesirably produce very corrosive solutions that may attack the material from which the equipment to be cleaned is made. For example, hydrochloric acid is commonly employed to remove iron oxide and copper oxides from equipment. Fe.sup.++ is generated by dissolution of iron oxide and is oxidized to the higher valance state Fe.sup.+++ by contact with the oxygen in the air. When Fe.sup.+++ comes into contact with base metal such as Fe.degree., corrosion occurs according to the following mechanism: EQU 2Fe.sup.+++ +Fe.degree..fwdarw.3Fe.sup.++
Further contact with the oxygen in the air generating the corrosive Fe.sup.+++ : EQU 3Fe.sup.++ +O.sub.2 .fwdarw.3Fe.sup.+++
Cu.sup.++ is stabilized with the C1.sup.- ion from hydrochloric acid, causing severe corrosion to copper according to the following mechanism: EQU Cu.sup.++ +Cu.degree..fwdarw.2Cu.sup.+
Contact with the oxygen in the air regenerating the corrosive Cu.sup.++ : EQU 2Cu.sup.+ +O.sub.2 .fwdarw.2Cu.sup.++
In order to avoid the corrosion associated with aeration of the solvent in the conventional process, nitrogen, an inert gas, may be substituted for standard air. Continuous use of nitrogen, however, adds significantly to the cost of the job. Reducing agents can also be used to reduce the corrosivity of the foamed solvent, but due to the "once-through" process, the cost is prohibitive. In addition, some industrial reducing agents such as stannous chloride, are themselves potential environmental hazards.
Foamed solvents, while very useful for cleaning, may also be difficult to control. It is not always possible to predict how quickly a vessel may be filled with foam. There may be overflow from the vessel which presents a risk of environmental contamination and exposure of workers to hazardous material. In addition to the spill hazard from collection tank overflow already discussed, there is a risk of air pollution. During a conventional foam breaking process, toxic or flammable vapors from volatile acids, organic stabilizers and reaction gases can be released to the atmosphere, creating air pollution, health hazards, and other safety risks. This is of particular concern when toxic or flammable gas is released into a closed building.
The need to inert a vessel is also a common industrial requirement. For example, it may be desirable to displace toxic or flammable gases that have been formed or collected in a closed vessel. Present methods of inerting generate undesirably large volumes of waste product or utilize vast expensive quantities of inert gas. In the former situation, water is generally employed to displace explosive vapors. The waste water contains hazardous material which must then be disposed of properly. In the latter case, continuous purging with an inert gas is cumbersome and costly.
The quality of the foam at any given time in the cleaning process also is a consideration. Foam quality is defined as the ratio of gas volume to the total volume or V.sub.g /V.sub.t X 100 or ##EQU1## The lower the quality the greater the moisture content. The higher the quality the lower the liquid content. "Wet foams" are defined as low quality foams while "dry foams" are of high quality. Foam is a compressible fluid; therefore (neglecting the solubility of gas in the solution and liquid expansion), the quality of foam at some other pressure can be calculated using Boyle's law. The ideal cleaning foam contains enough solvent in the liquid phase to continuously bathe the scale or foulant with solvent. Scale dissolution occurs in the liquid phase by a reaction of the solvent with the scale. A foam that is too dry (high quality) will not contain a sufficient amount of solvent for scale dissolution to occur, therefore, a good cleaning foam normally is equal to or less than "95" quality.
Thus, there has been a continuing need for a method for using and reusing foamed solvent which allows use of a lesser amount of inert gas where an inert gas is desirable or necessary, which allows for more efficient use of solvent, which eliminates the need for a large collection tank to break-down foam, which eliminates the need to break the foam down completely before refoaming, which allows for maintaining a foam of effective quality throughout the vessel to be cleaned, which can be conducted in a contained system to prevent the escape of hazardous or radioactive material, and which allows for control of the foam during the cleaning operation.
A method has now been found for using and recycling foamed solvents which obviates the problems of the conventional method discussed above. This new process may be employed in a closed or open system. It allows for recycling of foam and regeneration of natural break-down products of foam into new foam without danger of vessel overflow and subsequent ground pollution. It also allows recycling without danger of air pollution. In the herein disclosed process, recycling is accomplished by reuse of the foaming gas, reuse of the solvent, and reuse of the foamed liquid. It allows for less solvent to be used for a particular cleaning job. This process also reduces or eliminates the need for expensive or undesirable additives such as antifoaming agents, additives needed to refoam antifoam-contaminated liquid, and foam stabilizers.
The herein disclosed process is additionally advantageous as less personnel are required for performance of the cleaning job. Cleaning can be accomplished unattended once the process is set up since provision is made for foam control. This process can utilize both inert gas and reducing agents for corrosion control, but advantageously requires less inert gas and/or reducing chemicals because it provides for reuse of the gas and foamable liquid components. Expensive metering equipment for continuous blending of gas and liquid is also not required. The apparatus required for this new method can be assembled as a complete package on a single skid for use as a closed system or open system hookup.
It is further envisioned that the process will be especially useful for nuclear and environmental decontamination because reduced waste product from the cleaning operation is generated. This will reduce the need to dispose of large quantities of radioactive solvent. In addition, the radioactive equipment may be decontaminated with containment of hazardous materials because the process may be operated as a closed system.
In the process of inerting a vessel, foam may be generated from a fluid such as water or other suitable liquid, foaming agent, and an inert gas to displace undesirable vapors from a vessel. The foam initially displaces the vapors which are released through an opening in the vessel. Less waste is generated by the recycle of the foam and the continuous presence of foam in the vessel prevents additional vapors from forming from solvent that may have been contained in the vessel, and re-entry of air. This provides a safety factor when welding or other hot work must be performed in the vicinity of the vessel.